Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NOx storage catalyst which stores NOx which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges a fuel addition valve in the engine exhaust passage upstream of the NOx storage catalyst, which arranges an air-fuel ratio sensor inside the engine exhaust passage downstream of the NOx storage catalyst, and which uses the output signal of the air-fuel ratio sensor as the basis for feedback control of the air-fuel ratio of the exhaust gas which flows into the NOx storage catalyst to a target air-fuel ratio (see, for example, Patent Literature 1).
In this regard, it is known that this air-fuel ratio sensor gradually deteriorates if exposed to the heat of high temperature exhaust gas over a long time period or if poisoned by the HC in the exhaust gas, and as a result, the air-fuel ratio detected by the air-fuel ratio sensor can no longer accurately express the actual air-fuel ratio. In this regard, exhaust gas contains SOx. In this case, the air-fuel ratio sensor is believed to be affected by this SOx. However, how the air-fuel ratio detected by the air-fuel ratio sensor is affected by this SOx which is contained exhaust gas has not been known at all up to now.